Hazelcast: Merge two hazelcast instances

Question

Lets say we have two instances of hazelcast:

HazelcastInstance firstInstance = Hazelcast.newHazelcastInstance(new Config());
HazelcastInstance secondInstance = Hazelcast.newHazelcastInstance(new Config());
// Add entries to firstInstance
// Add entries to secondInstance

Now I am trying to delete everything from firstInstance and then add everything from secondInstance to firstInstance.

Is there a way to achieve this?

Answer 1

First, as per that initialization of the two instances shown in your code, both are cluster members belonging to the same cluster group and, given the default configuration, they will both contain all of the shared data. In other words, you wouldn't need to 'transfer' information.

If the above is true, by the time you're done deleting from the first instance, you won't have any copy of the data (other than their respective source).

If, however, the instances are initialized with configurations tying them to different cluster groups (remember, your code in the question isn't doing so), it's easy enough to just 'copy' using the Java Map/Collections API (which Hazelcast shared data structure types implement):

secondInstance.getMap("myMap").putAll(firstInstance.getMap("myMap"));
firstInstance.getMap("myMap").clear(); //please confirm this.

Distributed lists can be treated in a similar manner. Also, be careful with such 'bulk copies' as your member can hit an out of memory error (of course, that depends on the size of your data).

More about this can be read here: http://docs.hazelcast.org/docs/3.6/manual/html-single/index.html#preventing-out-of-memory-exceptions